In a telecommunications system, a radio base station is used for transmitting information to a mobile station and for receiving information from a mobile station. The information transmitted or received is modulated onto a carrier signal. A carrier signal is a signal of a specific nominal frequency upon which one or more channels for communicating information between a base station and one or more mobile stations are defined. Other channels are defined at other carrier signals of other nominal frequencies. For a mobile to be able to receive information transmitted on a channel, it is important that the base station transmits at the expected frequency. For the base station to be able to receive information transmitted from a mobile over a channel of a specific carrier frequency, it must receive at the expected frequency. Also, for the different channels not to disturb each other, it is important that the carrier frequencies do not deviate from their center frequencies by more than a tolerable amount.
According to the IS-54 air interface standard, the center frequency of a carrier signal generated by a base station is required to vary by at most 0.25 ppm.
Carrier signals may be generated in a synthesizer module in the radio base station. To meet the stability requirements according to e.g., IS-54, an oscillator of high stability regarding the amount of variance, or drift, of center frequency of the generated oscillating signal can be used as a synthesizer reference from which the carrier signals are synthesized. The key issue is how to make this synthesizer reference stable enough. A synthesizer module synthesizes carrier signals of selected frequencies. Because the oscillating signal generated by the oscillator forms the synthesizer reference, the stability of the carrier signal is dependent upon the stability of the oscillator.
One way to provide a synthesizer reference used in today's systems is to use an oscillator in a free-wheeling (or free-running) mode. However, such a solution has several disadvantages. Either the oscillator (Rubidium) becomes very expensive, power-consumptive, large and heavy and operable in a limited temperature range, or the oscillator (OCXO) is susceptible to aging, which means that it has to be calibrated regularly (manually, unless a stable reference can be provided).
Another existing solution is to lock the oscillator in a phase lock relationship to a long-term stable reference clock signal, as shown in FIG. 1. In FIG. 1, a PCM link clock signal is fed into a phase detector 1 of an ordinary phase-locked loop by way of a clock extractor 2. The phase detector detects differences in phase between the PCM link clock signal and an output signal generated by an oscillator 3 in a feedback connection. An output of the phase detector is fed into a low pass filter 4 which filters low-frequency components of the output of the phase detector provided thereto. The filter output formed by the filter is fed into the oscillator 3 as a reference clock signal from which the oscillator generates the output signal which is then used as a synthesizer reference clock signal by a radio part, here represented by a modulator and synthesizer 5. As noted above, the output signal is also fed back into the phase detector 1 in a feedback connection.
This solution used in some conventional systems, avoids the need for calibrating the oscillator for aging, by designing it to run phase locked to a long-term stable reference clock signal. The reference clock signal can be extracted from a long-term stable clock signal distributed on the PCM link which is used for transmission of information between the base station and a mobile switching center (MSC) in the mobile network. This signal is susceptible to jitter and wander, and holdover.
The long-term stable clock signal provided on the PCM link is generated in a signal chain from, for example, an atomic clock. Holdover occurs when the signal chain between the atomic clock signal and the long-term stable clock signal on the PCM link is broken, for example because of a restart in a switch (e.g., an MSC) in the chain. When this happens the center frequency of the PCM link clock signal may deviate from a nominal center frequency by some amount. Note that this means that although the signal chain is broken for a period pf time, the PCM link clock signal is still present, but may be out of range. Holdover occurs during relatively short periods (e.g., less than typically one hour).
In FIG. 2, a PCM link clock signal subjected to jitter 6 (&gt;1 Hz), wander 7 (&lt;1 Hz) and holdover 8 is shown. &lt;D&gt; indicates the center frequency deviation due to holdover 8.
When running phase-locked to such a signal, the conventional phase-locked loop will generate an output signal 9 (to be used as a synthesizer reference signal) as shown in FIG. 3. Therein it can be seen that a certain influence from wander and holdover occurring on the PCM link is propagated through the phase-locked loop. &lt;x&gt; denotes the maximum deviation in frequency of the synthesizer reference signal, which is required to stay within certain limits (0.25 ppm for IS-54) for the stability requirement of the air interface standard to be fulfilled.
The quality of the clock signal is measured in Stratum levels. The Stratum level of a PCM link clock specifies maximal allowed frequency deviation due to holdover. To ensure that the stability requirement 0.25 ppm (which is of the order 10.sup.-7) for the carrier frequency is not violated (which means that &lt;x&gt; in FIG. 3 must be less than 0.25 ppm), the frequency deviation of the reference clock must also stay within these limits. This is true for a PCM link clock of Stratum-2 level (or less), for which frequency deviation due to holdover (&lt;D&gt; in FIG. 2) stays within 1.8 10.sup.-8. However, such a link is expensive. It would be desirable to use a cheaper link of quality Stratum-3 (or even Stratum-4).
For links (clocks) of Stratum-3 level or worse, frequency deviation during holdover mode (&lt;D&gt;in FIG. 2) does not stay within the required limits as the frequency deviation &lt;D&gt; in this case is only required to stay within 4.6.times.10.sup.-6 ppm.